# Full text of "Text Book Of Mechanical Engineering"

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```Rectangular Shafts.

421

The Strength of a Rectangular Section was given by
St. Venant as follows :

Modulus of resistance of rectangular section = '294 4 -7^  -
while the pure ring theory would give    *i666 b h ^W-^TP, and

the discrepancy increases with the ratio T
Thus if £=i"and h=*2n

Tm=*5266 ton ins. (i) by St. Venant; and 745 ton ins.

[(2) by ring theory,
and (i)  7 of the diagram value (2).

If   £=i" and & = 4"
Tm = 1*142 (i) and 2747 (2) respectively
or (i) = '41 of diagram value.

Hexagonal shafts may be treated directly from diagram.

Strength  of Shafts by   Direct   Experiment.  The

following experimental figures may be used by way of correction.
Moment of any shaft d^ or s = Figure in table x (d* or s3).

BREAKING MOMENTS OF SHAFTS i" DIA. AND i" SQUARE,
IN TON-INCHES (.ELSWICK EXPERIMENTS).

Round.
Square.

Wrought Iron ..... .    ......
T^c;
6 -81

Cast Iron ........... .
D Oj \"?I
uo 6*78

Steel ..................
8-02
11*6

Yellow Brass  ............
2 'AC
^*i <

Cast Copper   . . .....
* *rj
2'ic;
o A 0 2'74

A factor of 10 is to be used for short shafts and of 16 for long
shafts, to secure stiffness. Strength is rarely the sole criterion.

Example 17.Find the relative weights of two shafts of equal
strength; the one solid, and the other hollow, with a hole half the out-
side diameter. (Eng. Exam. 1892.)```